Comets are small, fragile, irregularly shaped bodies composed mostly of a
mixture of water ice (ice composed of H20), dust, and carbon- and silicon-based
compounds. They have highly elliptical orbits that repeatedly bring them very
close to the Sun and then swing them into space. Comets have three distinct
parts: a nucleus, a coma and a tail. The solid core is called the nucleus,
which develops a coma with one or more tails when a comet sweeps close to
the Sun. The coma is the dusty, fuzzy cloud around the nucleus of a comet,
and the tail extends from the comet and points away from the Sun. The coma
and tails of a comet appear only when the comet is near the Sun.